The present invention relates to a method for producing an electrophotographic recording material composed of a double layer of amorphous and crystallized selenium which is applied to an electrically conductive substrate.
Electrophotographic processes and apparatus for this purpose have found wide acceptance in the duplicating art. They are based on the property of a photoconductive material to change its electrical resistance when exposed to an activating radiation.
After electrically charging and illuminating a photoconductive layer with an activating radiation, a latent electrical charge image corresponding to an optical image can be produced on the photoconductive layer. At the illuminated points, the conductivity of the photoconductive layer is increased to such an extent that the electrical charge can flow off, at least in part, through the conductive substrate; or in any case more than at the unilluminated points. At the unilluminated points, the electrical charge remains essentially unchanged. The image can be made visible by means of an image powder, a so-called toner, and the resulting toner image can then be transferred, if desired, to paper or to some other medium.
Electrophotographically effective substances employed are organic as well as inorganic. Among these substances, selenium, selenium alloys and selenium compounds have gained particular significance. Especially in the amorphous state, these substances play an important part and have found many fields of use.
The change in electrical conductivity of a photoconductive layer depends on the intensity of the wavelength of the radiation employed. In the range of visible light, which is preferred for practical use in electrophotography, amorphous selenium exhibits high sensitivity on the blue side (the short-wave region), but only very slight sensitivity on the red side (the long-wave region).
The result is that an electrophotograph displays a red character in the same way as a black character. Under certain circumstances, particularly when the master is in color, this result may be a drawback in practical use. For example, a black character on a red background (or vice versa) cannot be distinguished from its background and, therefore, cannot be identified.
It is known that crystallized selenium, as opposed to amorphous selenium, is extremely red sensitive, or "high dark conductive". Therefore, when crystallized selenium is used, that part of the visible spectrum above 650 nm can also be utilized. However, the high dark conductivity of crystallized selenium, i.e., its property of conducting electrical current already in the unilluminated state to such a degree that an electrical charge applied to its surface cannot be held for the length of time required for electrophotographic purposes, also weighs against its use in electrophotography.
A photoconductive layer material which is red and blue sensitive, and is simultaneously distinguished by low dark conductivity, exists in a combination of amorphous and crystallized selenium. The combination of these two forms of selenium may have a layer structure where, for example, crystallized selenium is initially applied to a conductive substrate upon which a layer of amorphous selenium is then applied.
However, the manufacture of such a dual photoconductive layer is very difficult because in order to assure perfect operation, i.e., particularly to provide uniform red sensitivity over the entire area of the photoconductive layer, it is necessary for the lower partial layer to be crystallized to cover the entire surface area with a completely uniform and sufficient thickness.
However, such uniformity is difficult or impossible to attain during or after the vapor-deposition process under the influence of the higher temperatures required to produce crystallization pursuant to the conventional thermal treatment of a vapor-deposited amorphous layer. This is due to the minimum required temperature of about 70.degree. C. to produce crystallization and the competing requirement that the temperature be kept as low as possible to attain the greatest possible uniformity during crystallization. For these reasons, a very narrow constant temperature range must be set during vapor-deposition which involves considerable expenditures, particularly for the manufacture of larger quantities, and especially since this temperature regulation must take place in a vacuum.
Moreover, crystallization depends to a great degree on other crystal nuclei, such as surface inhomogeneities, which may happen to be present on the substrate surface. Spontaneous crystallization resulting from these nuclei is almost impossible to control, especially since the crystallization depends on the variables of the individual substrate. With such irregular crystal nuclei formation, the further development of the crystallization cannot be performed reliably with the required uniformity.
However, if temperatures are increased to assure with greater certainty the complete crystallization of the lower partial layer, there exists the danger that the upper partial layer, which must retain its amorphous state in order to maintain its low dark discharging capability, will also convert (more or less, and irregularly) to the crystallized state--which is just as undesirable.
A publication by Kohei Kiyota and Kunihiko Tasai, entitled "Selenium Element for Photoelectrostatography", published in the Fujitsu Scientific and Technical Journal (December, 1975), discloses that a photoconductive dual layer of crystallized and amorphous selenium can be produced by vapor-deposition of selenium onto a layer of manganese which acts as a crystallization nucleus for the selenium. The conversion of the initially amorphous selenium to the crystallized form disclosed in this publication is effected by subsequent tempering at about 80.degree. C. to keep the conversion rate high enough so that the conversion time is sufficiently short. The drawback in selecting such a high conversion temperature is the previously-described danger that the upper amorphous partial layer will then also convert to the crystallized state. Therefore, the process disclosed in this publication also requires the maintenance of a very narrowly defined temperature range. Moreover, the additional process step of tempering after the vapor-deposition step is still necessary.
German Auslegeschrift No. 2,808,757 (Application published after examination) (which corresponds to our pending Application Ser. No. 012,858) discloses a process in which a dual layer of amorphous and crystallized selenium is produced on a conductive substrate after tellurium is applied to the conductive substrate to a layer thickness of 10 to 500 nanometers. The electrophotographic recording material produced in this way is distinguished by improved spectral sensitivity.